In machines utilizing the electrophotographic process, as in copier machines, there is usually a magnetic roll that attracts magnetizable particles called beads. These beads attract toner particles which become known as the carrier mix. As the carrier mix is magnetically attracted to the magnetic roll, which is essentially a cylinder with magnets inside, the surface of an electrostatically charged photoconductor passes in close proximity to the surface of the magnetic roll. Utilizing the principles of electrostatics and electrophotography, the toner on the magnetic roll is transferred to the photoconductor in a predetermined pattern which corresponds to the image pattern of the original to be copied or reproduced. The toner that is not adhered to the photoconductor during the image transfer ideally returns to the developer sump containing the carrier mix to await attraction to the magnetic roll.
As a practical result, however, the toner dust and beads have a tendency to migrate to undesired areas throughout the electrophotographic development device. Although this migration occurs in any type of copier development device, it is especially evident in a two cycle copier development process.
A typical two cycle process is described in U.S. Pat. No. 3,647,293 to Queener, and assigned to the same assignee as this application. As in any process, either one or two cycles, the two cycle process has the usual facilities for charging, imaging, developing, cleaning, transferring, precleaning, and erasing during the process of producing copies from an original document. However, in a two cycle process there is a combined unit for performing both the developing and the cleaning functions at a single station in a proper timed sequence. The combined unit incorporates the magnetic brush roll with a biasing means to establish the appropriate bias to initiate transfer of toner onto the photoconductor surface during the developing step, and to attract residual toner from the photoconductor surface during the cleaning operation. Since the magnetic brush roll serves the dual role of cleaning and developing, there is no need for a separate cleaning station with a conventional cleaning brush. Nevertheless, there may be an unwanted toner dust cloud that needs to be controlled from this developing and cleaning process.
In order to control the toner dust and beads from unwanted migration, it has been known heretofore to utilize a seal. These seals have typically been of the contacting type, and usually of materials similar to foam or other materials having resilient properties. These contacting seals use a wiping action against a rotating surface to create a seal.
However, these type of contacting seals have a tendency to cause the formation of clinkers. Clinkers are chunks of toner that have been mechanically compressed by a wiper or blade pressing against the toner on the photoconductor. These clinkers, or oversized toner particles, may advance through the development process onto the photoconductor during imaging. Consequently, as the paper comes into contact with the photoconductor for the image transfer, the clinker, due to its size, will stand the paper away from the photoconductor at that locality. As a result, the image on the paper will not develop out evenly, making a less than perfect copy of the original image.
Furthermore, it is known to place these contacting seals between the developer side plate and the magnetic roll to prevent the toner dust and beads from migrating to the bearings of the magnetic roll, or between the developer end plate and the photoconductor such that the foam material presses against the photoconductor forming a seal.
In electrophotographic machines that utilize the above mentioned contact seals, the end plate for the magnetic roll, or the developer side plate, is not in alignment with the end plate that mounts the photoconductor drum. This is due to the inherent critical positional relationship in any electrophotographic development device between the photoconductor and the magnetic roll. For instance, the carrier mix must extend to the edge of the largest image desired for an accurate reproduction of an original in a copy machine. No roll-off in the amount of the carrier mix can be tolerated within the image area. As a consequence, the magnets in the rotating magnetic brush roll must extend some distance beyond the image edge to insure there is no carrier mix roll-off prior to the image edge.
In addition, the coronas have a relationship that must be taken into account. The charge corona must charge all of the image area and must extend somewhat beyond the image area. The preclean corona must be capable of neutralizing the entire image area but cannot be quite as wide as the charge corona. Any other corona of a positive polarity that might be in any particular machine must also be of lesser width than the charge corona. Because of these corona relationships, there must be a photoconductor area extending beyond the image edge of sufficient magnitude to provide a suitable tolerancing of the corona edges. Since the magnetic brush roll must also extend beyond the image edge, the amount of photoconductor beyond the image edge can increase to a significant degree. Adding to that, if a seal for the developer is placed in contact with the photoconductor, still more photoconductor area must be added to the size of the unit.